[TTI] Fix getGEPCost() for geps with a single operand

[davide]

Summary

Previously this would sporadically crash as TargetType was never initialized.
I just mimiced what the basic model does here, marking it as free. I'm happy to change this to Basic if folks think it's more appropriate.

[davide]

include/llvm/Analysis/TargetTransformInfoImpl.h

677

Note, this is not strictly part of the fix (but it's needed to trigger it deterministically, as on my machine the uninitialized pointer get weird values).

[fhahn]

LGTM. Thanks for addressing this issue, this is definitely an improvement and much better than passing an uninitialized value to isLegalAddressingMode.

I think to mirror the behavior of isLegalAddressingMode in this file as closely as possible, we should return TTI::TCC_Free, if !BaseGV, else TTI::TCC_Basic.

The other minor nit is just about the position of the early exit, and I think either position is fine.

include/llvm/Analysis/TargetTransformInfoImpl.h

706

If I understand correctly, we use this early exit to cover the case where TargetType is NULL, because we didn't execute any iterations of the loop above.

I think it would be slightly clearer to just move the early exit before TargetType, with a comment that this covers the case where TargetType would be null.

include/llvm/Analysis/TargetTransformInfoImpl.h

              "explicit pointee type doesn't match operand's pointee type");
       BaseGV = dyn_cast<GlobalValue>(Ptr->stripPointerCasts());
     }
     bool HasBaseReg = (BaseGV == nullptr);
     int64_t BaseOffset = 0;
     int64_t Scale = 0;
 
     auto GTI = gep_type_begin(PointeeType, Operands);
-    Type *TargetType;
+    Type *TargetType = nullptr;
     for (auto I = Operands.begin(); I != Operands.end(); ++I, ++GTI) {
       TargetType = GTI.getIndexedType();
       // We assume that the cost of Scalar GEP with constant index and the
       // cost of Vector GEP with splat constant index are the same.
       const ConstantInt *ConstIdx = dyn_cast<ConstantInt>(*I);
       if (!ConstIdx)
         if (auto Splat = getSplatValue(*I))
           ConstIdx = dyn_cast<ConstantInt>(Splat);

           if (Scale != 0)
             // No addressing mode takes two scale registers.
             return TTI::TCC_Basic;
           Scale = ElementSize;
         }
       }
     }
 
+    // If this GEP has a single operand, the basis, it should be cheap.
+    if (Operands.empty())
+      return TTI::TCC_Free;
+
     // Assumes the address space is 0 when Ptr is nullptr.
     unsigned AS =
         (Ptr == nullptr ? 0 : Ptr->getType()->getPointerAddressSpace());
     if (static_cast<T *>(this)->isLegalAddressingMode(
             TargetType, const_cast<GlobalValue *>(BaseGV), BaseOffset,
             HasBaseReg, Scale, AS))
       return TTI::TCC_Free;
     return TTI::TCC_Basic;

test/Transforms/SimplifyCFG/gepcost.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -simplifycfg | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "thumbv7m-none--eabi"
+
+@glob = external unnamed_addr constant [16 x i8]
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entr:
+; CHECK-NEXT:    br i1 undef, label [[NEXT:%.*]], label [[EXIT:%.*]]
+; CHECK:       next:
+; CHECK-NEXT:    [[PAT:%.*]] = getelementptr [16 x i8], [16 x i8]* @glob
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entr:
+  br i1 undef, label %next, label %exit
+
+next:
+  %pat = getelementptr [16 x i8], [16 x i8]* @glob
+  br label %exit
+
+exit:
+  ret void
+}